fix duplicate and recursive footnotes. (#241)

Fix the infinite loop when there is a self-refer footnote by checking if
the reference is already added into parser.notes.

It also fixes of creating duplicate footnotes through this modification.

Add coresponding testcase.

inline.go

@@ -498,7 +498,7 @@ func link(p *parser, out *bytes.Buffer, data []byte, offset int) int {
 				return 0
 			}
 
-			if t == linkDeferredFootnote {
+			if t == linkDeferredFootnote && !p.isFootnote(lr) {
 				lr.noteId = len(p.notes) + 1
 				p.notes = append(p.notes, lr)
 			}

inline_test.go

@@ -1023,6 +1023,28 @@ what happens here
 </li>
 </ol>
 </div>
+`,
+	`testing footnotes.[^a]
+
+test footnotes the second.[^b]
+
+[^a]: This is the first note[^a].
+[^b]: this is the second note.[^a]
+`,
+	`<p>testing footnotes.<sup class="footnote-ref" id="fnref:a"><a rel="footnote" href="#fn:a">1</a></sup></p>
+
+<p>test footnotes the second.<sup class="footnote-ref" id="fnref:b"><a rel="footnote" href="#fn:b">2</a></sup></p>
+<div class="footnotes">
+
+<hr />
+
+<ol>
+<li id="fn:a">This is the first note<sup class="footnote-ref" id="fnref:a"><a rel="footnote" href="#fn:a">1</a></sup>.
+</li>
+<li id="fn:b">this is the second note.<sup class="footnote-ref" id="fnref:a"><a rel="footnote" href="#fn:a">1</a></sup>
+</li>
+</ol>
+</div>
 `,
 }
 
@@ -1076,6 +1098,34 @@ func TestNestedFootnotes(t *testing.T) {
 </li>
 </ol>
 </div>
+`,
+		`This uses footnote A.[^A]
+
+This uses footnote C.[^C]
+
+[^A]:
+  A note. use itself.[^A]
+[^B]:
+  B note, uses A to test duplicate.[^A]
+[^C]:
+  C note, uses B.[^B]
+`,
+		`<p>This uses footnote A.<sup class="footnote-ref" id="fnref:A"><a rel="footnote" href="#fn:A">1</a></sup></p>
+
+<p>This uses footnote C.<sup class="footnote-ref" id="fnref:C"><a rel="footnote" href="#fn:C">2</a></sup></p>
+<div class="footnotes">
+
+<hr />
+
+<ol>
+<li id="fn:A">A note. use itself.<sup class="footnote-ref" id="fnref:A"><a rel="footnote" href="#fn:A">1</a></sup>
+</li>
+<li id="fn:C">C note, uses B.<sup class="footnote-ref" id="fnref:B"><a rel="footnote" href="#fn:B">3</a></sup>
+</li>
+<li id="fn:B">B note, uses A to test duplicate.<sup class="footnote-ref" id="fnref:A"><a rel="footnote" href="#fn:A">1</a></sup>
+</li>
+</ol>
+</div>
 `,
 	}
 	doTestsInlineParam(t, tests, Options{Extensions: EXTENSION_FOOTNOTES}, 0,

markdown.go

@@ -241,6 +241,16 @@ func (p *parser) getRef(refid string) (ref *reference, found bool) {
 	return ref, found
 }
 
+func (p *parser) isFootnote(ref *reference) bool {
+	for _, v := range p.notes {
+		if string(ref.link) == string(v.link) {
+			return true
+		}
+	}
+
+	return false
+}
+
 //
 //
 // Public interface